In Japan, photovoltaic power generation systems are now being commercially introduced and coming into widespread use. Although some p-n junction silicon solar cells are available on the market, their electricity prices are much higher than those of distribution lines. Accordingly, inexpensive solar cells are required for the proliferation of photovoltaic power generation systems. FIG. 3 is a schematic diagram showing a cross-sectional structure of a conventional dye-sensitized solar cell. Reference numeral 101 represents a glass substrate, and reference numeral 102 represents an electrode that is provided on the lower surface of the glass substrate 101 and that is composed of a transparent material, for example, tin oxide (SnO2) doped with fluorine (F). Reference numeral 103 represents a light-absorbing layer of fine semiconductor particles 103a deposited on the electrode 102 and composed of titanium oxide with a particle size of about 50 nm or less, dye 103b being adsorbed on the surfaces of the fine semiconductor particles. The light-absorbing layer 103 is formed in a film about 10 μm or less in thickness. An electrolyte 104 includes or infiltrates the light-absorbing layer 103. Reference numeral 105 represents a counter-electrode.
Such a dye-sensitized solar cell, as compared with a semiconductor p-n junction solar cell, has an advantage that it can convert light at longer wavelengths into electricity. In addition, the dye-sensitized solar cell has another advantage in that it can be manufactured using less energy than the p-n junction solar cell, which requires considerable energy for its manufacture.
While the dye-sensitized solar cell has such advantages, however, its low conversion efficiency stands in the way of commercialization. This low conversion efficiency is caused by high transmission of light of long wavelengths in the light-absorbing layer 103 because of the very low light absorptivity of the dye 103b. To improve the conversion efficiency, the light-absorbing layer 103 composed of the fine semiconductor particles 103a carrying dye 103b may be increased in thickness. However, a thicker light-absorbing layer 103 will cause a problem of further reduced conversion efficiency, because the light-absorbing layer 103 structurally has high series resistance.
In contrast, reducing the thickness of the light-absorbing layer 103 to decrease the series resistance will increase the transmission of long-wavelength light, lowering the light absorption efficiency. In both cases, therefore, it is difficult to increase the conversion efficiency. Consequently, a dye-sensitized solar cell that satisfies these contradictive requirements for conversion efficiency and light absorption efficiency as described above has not yet been developed.
In light of the existing problems described above, an object of the present invention is to provide a dye-sensitized solar cell with high conversion efficiency.